This invention relates to a transmission control system, and more particularly to a calibrating method for determining key parameters relating to the operation and control of the transmission control clutches.
Some manufacturers have used versions of electrohydraulic transmission controls which include proportionally controlled valves. In such systems with proportional control valves it is possible and desirable to accurately control the torque capacities of the clutches during engagement. While the electrical command supplied to the control valve may be very precise, manufacturing tolerances in the valves and transmission cause large variations on an actual vehicle. If it is known what electrical command corresponds to the initial clutch engagement pressure which causes a clutch to just begin carrying torque, then this command could be used to modify the shift command for that clutch during shifting to provide optimized control. For example, U.S. Pat. No. 4,855,913, issued Aug. 8, 1989 to Brekkestran et al., discloses that the key parameters in the control system include the initial clutch engagement pressure (represented by DC-MAX) and the fast-fill clutch delay (represented by T1). The Brekkestran reference further states that DC-MAX and T1 must be determined by laboratory or field tests. However, the Brekkestran reference does not disclose any method for determining these values.
A calibrating method or a method of determining the pressure necessary to achieve clutch engagement in a microprocessor-based transmission control system is described in U.S. Pat. No. 4,989,471, issued on Feb. 5, 1991 to Bulgrien. The Bulgrien method includes braking the transmission output shaft, then gradually increasing the clutch pressure and saving a value corresponding to the clutch pressure at which the engine speed begins to decrease. However, this method relies upon the resistance to rotation due to use of the vehicle brakes to prevent rotation of the transmission output shaft. Such a procedure could be dangerous if the vehicle brakes are not applied or if the brakes fail, because then undesired vehicle motion could result during calibration. The Bulgrien patent also illustrates an alternate method of calibrating a clutch by sensing when the clutch transmits sufficient torque to move the vehicle. This alternate method requires that the vehicle be placed in a position where vehicle motion is not a safety concern, and the results of such a method will vary depending upon the terrain on which the vehicle is placed. The Bulgrien patent also depends on sensing variations in engine speed, and is therefore susceptible to variations in engines and engine governors.
U.S. Pat. No. 5,082,097, issued on Jan. 21, 1992 to Goeckner et al. discloses a calibrating system or a system for determining a current signal corresponding to the point at which the clutch begins to transmit torque. This system requires sensing either vehicle movement or engine speed droop, and thus depends on sensing variations in engine speed, and is therefore susceptible to variations in engines and engine governors, or requires possibly dangerous vehicle movement.
Another calibration method is described in U.S. Pat. No. 5,224,577, issued Jun. 7, 1993 to Falck et al. and assigned to the assignee of the present application. This method also involves sensing engine speed droop, and is therefore susceptible to variations in engines and engine governors.
Another calibration method is disclosed in U.S. Pat. No. 5,337,871, issued Aug. 16, 1994 to Testerman, and assigned to the assignee of the present application. However, this method requires pressure sensors, which are expensive, and which are not as accurate or reliable as rotation speed sensors.
Another calibration method is disclosed in U.S. patent application Ser. No. 08/800,431, filed Feb. 8, 1997 now U.S. Pat. No. 5,842,375, and assigned to the assignee of the present application. In this method the target deceleration time used for determining the hold pressure must be determined empirically as an average based on measurements taken from a number of sample transmission. However, if the actual parasitic drag of a production transmission is significantly different from that of the sample transmissions, then the torque produced by the resulting hold pressure of the clutch being calibrated would be different from what is desired. Accordingly, it would be desirable to measure a parasitic drag time for each transmission prior to determining the hold pressure of each clutch, and then use the actual parasitic drag time to calculate the target deceleration time required to produce a given hold torque.